1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to a method of fabricating a reticle with customized pattern generation.
2. Description of the Related Art
The minimum possible horizontal feature size of a structure patterned on a semiconductor wafer in conventional processing is dictated, in large part, by the resolution of the photolithographic system used to pattern the wafer. The resolution of an optical photolithographic system is normally an aggregate of a number of physical mechanisms, such as lens aberrations, light spectrum, and diffraction effects. However, diffraction effects constitute the dominant limitation to resolution, particularly in projection printers, and particularly in sub-1.0 .mu.m processing.
All optical lithography systems direct electromagnetic radiation past various edges and through various slits, and thus involve light diffraction to one degree or another. The edges and slits are natural features of the patterns of polygonal structures on conventional reticles and masks, and may number in the hundreds, thousands, or even millions, depending on the complexity of the reticle. The general effect of diffraction is a spreading of the radiation into regions that are not directly exposed to the oncoming waves. A more localized effect is observed where radiation passes adjacent angled surfaces of two adjacent polygonal structures of the reticle. The inside corner of an L-shaped polygon and the outside corner of an adjacent L-shaped polygon represent one example of such adjacent angled surfaces. The diffraction pattern produced by the interaction of radiation with the adjacent angled surfaces is highly complex and can be difficult to accurately characterize. However, the effects of the complex diffraction pattern may be readily observed by inspecting a resist layer patterned with the reticle. The images of the adjacent L-shaped polygon structures patterned in the resist may exhibit a pronounced bulging in the vicinity of the adjacent angled surfaces, i.e., the adjacent inside and outside corners. This bulging may be extensive enough to actually merge the images of the polygon structures in the resist. Even without merging, the bulging may be extensive enough to cause a bridging of the device structure that is subsequently patterned with the resist. Such bridging can lead to a host of yield-limiting problems, such as shorts and deviations from design rules, to name just a few.
The fabrication of increasingly smaller features in integrated circuits is dependent on the availability of increasingly higher resolution optical lithography equipment. Designers of optical lithography equipment have employed several techniques to combat the deleterious effects of light diffraction. Some of these techniques include decreasing the wave length of the illuminating light, increasing the numerical aperture of the system, increasing the contrast of the resist by modifying resist chemistry or by creating entirely new resists, and adjusting the coherence of the optical system. Even with the availability of these various techniques for increasing the resolution of optical lithography equipment, the best of conventional optical lithography systems have a resolution limit of about 0.2 .mu.m , and may still produce images with bridged patterns.
Electron beam or "e-beam" lithography has occasionally been used as a substitute for optical lithography in circumstances where the resolution limits of the prevailing optical lithography techniques prevented successful fabrication of a particular integrated circuit. However, there are several drawbacks associated with electron beam lithography, including resolution limitations associated with electrons forward scattered in the resist and back scattered from the substrate, swelling, which often occurs during development of a negative e-beam resist, extremely slow processing times when compared to optical projection systems, and significantly higher cost of electron beam lithography systems compared to optical steppers.
X-ray lithography has also been used occasionally in place of optical lithography to obtain resolutions in the sub-1.0 .mu.m area. However, as with electron beam lithography, certain technical difficulties have prevented X-ray lithography from supplanting optical lithography as the lithographic process of choice in mass produced integrated circuits. X-ray reticles have proven to be extremely difficult to reliably manufacture and throughput during imaging is notoriously slow.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.